The present invention relates to an electrical insulating layer (hereinafter referred to as an insulating layer) having excellent heat resistance and a low dielectric constant, exhibiting low water absorption, and capable of being prepared from relatively inexpensive starting materials. The insulating layer of the present invention is useful as an insulating substrate for printed circuit boards and as an insulating film for various purposes in semiconductor devices.
Printed circuit boards comprising an insulating substrate having an electrically conductive printed pattern on the surface thereof have been widely used as substrates for mounting electronic devices. Printed circuit boards are classified into rigid boards and flexible boards. Both types have a substrate which is usually made of a resin or resinous material. Substrates for use in rigid printed circuit boards are usually prepared by impregnating a base material such as paper, glass cloth, or glass mat with a prepolymer of a thermosetting resin such as a phenolic resin, epoxy resin, or unsaturated polyester resin, drying the impregnated base material to form a prepreg, and laminating a plurality of plies of the prepreg to form a laminated sheet. Flexible printed circuit boards, which were developed to respond to a demand for electronic devices of smaller dimensions, have substrates comprised merely of a flexible film of a thermosetting resin.
With an increase in packaging density in recent years, insulating substrates are required to have an increased heat resistance, particularly with respect to long-term heat resistance, and a decreased dielectric constant. In order to meet these requirements, a polyimide resin which is a typical heat-resistant thermosetting resin has been used to produce an insulating substrate including a flexible substrate and a rigid substrate based on a glass cloth or glass mat.
An insulating layer made of a resin is also used as an insulating film for various purposes in semiconductor devices. Such insulating films include interfacial insulating films for electrically separating adjacent layers in a multilayered printed circuit board, passivation films used to cover p-n junction surfaces for stabilization, buffer films for protecting a semiconductor device from applied forces, and .alpha.-ray shielding films used in DRAM's and high-speed bipolar IC's. Recently, an insulating film has been used in a multichip module (MCM).
Due to heat generation during the operation of a semiconductor device, an insulating film for use in semiconductor devices is required to have good long-term heat resistance in addition to good insulating properties including a low dielectric constant and a high resistivity. It is also desirable to have good mechanical properties and chemical resistance. In order to meet these requirements, interfacial films and passivation films have been formed mostly from an inorganic insulating substance such as silicon dioxide or glass in the prior art. However, there is a recent trend to use a polyimide resin to form an insulating film for semiconductor devices. An insulating film made of an organic resin is also useful as a buffer film or an .alpha.-ray shielding film. Such a film can be formed by coating with a resin solution followed by drying or baking, which is a simpler process than the process used to form an inorganic film which includes sintering at a high temperature or a chemical reaction such as oxidation.
It is known that a polyimide resin has excellent heat resistance and can withstand a high temperature above 300.degree. C. However, such a temperature is the maximum short-term heat resisting temperature, and the maximum long-term heat resisting temperature of the resin is as low as about 200.degree. C., although it is still much higher than that of an epoxy resin (about 150.degree. C. or lower). A polyimide resin also has a low dielectric constant in the range of 3.5-3.7, a high volume resistivity on the order of 10.sup.16 -10.sup.17 .OMEGA.-cm at room temperature, and good chemical resistance. Therefore, among commercially available resins, a polyimide resin is regarded as the material having the highest performance both for substrates of printed circuit boards and for insulating films for semiconductor devices.
However, polyimide resin films are expensive, and they have a relatively high water absorption on the order of 3% as measured by immersion in water for 24 hours at 25.degree. C. due to a high polarity of imide groups. Water absorption may cause the films to blister, which may in turn produce shear of the circuit formed thereon or even a short circuit. The dielectric constant of polyimide films increases to 4.5-4.7 when glass fibers are present therein as in the case of rigid substrates.